Extremely short duration optical pulses are important for high speed signal processing and communications. Optical pulses in this category are commonly called ultrashort optical pulses and have durations in the picosecond and sub-picosecond range. Ultrashort optical pulses have been obtained by techniques such as active modelocking, for example, by changing the transmission of a modulator disposed within the lasing cavity. As an alternative to active modelocking techniques, ultrashort optical pulses also have been obtained by techniques such as passive modelocking wherein a nonlinear element, such as a saturable absorber, is placed either within the lasing cavity or within a cavity external to the lasing cavity.
Active modulators, such as p-i-n modulators, actively mode-lock lasing operation by introducing periodic loss modulation to a laser cavity. Periodic loss modulation is effected by changing a voltage applied to the modulator (which is disposed within the laser cavity) to change the reflectivity of the modulator. Because the laser operates at the lowest loss state, which corresponds to the highest reflectivity of the modulator, all the axial modes of the laser are locked together when the modulator has the highest reflectivity, which then produces short pulses in the time domain.
Saturable absorbers are materials which display a change in opacity related to the incident radiation intensity at a particular wavelength. Solids, liquids, and gasses may be used as saturable absorbers based upon the chosen wavelength of operation. The saturable absorber acts as a shutter. It absorbs all weak radiation incident upon it. As the intensity of the incident radiation reaches a sufficiently high level, called the "saturation intensity" of the saturable absorber, the incident radiation is permitted to pass through the absorber. In general, the attenuation of the incident radiation caused by the saturable absorber is relatively low because the absorber is saturated into a transparent state at the wavelength for the incident radiation.